1. Field of the Invention
The present invention relates to a water-circulating type ice maker having a refrigerating circuit comprising an ice-making water circuit where ice-making water is circulated and supplied to an ice-making plate by a circulating pump, an evaporator provided at the ice-making plate for freezing the circulating water on the ice-making plate, and a condenser which is cooled by the air surrounding the ice maker or a cooling medium such as cooling water; in particular, the present invention relates to a device capable of preventing the phenomenon of generating cotton-like or slushy partial ice, which are characteristic of this type of an ice maker, (hereinafter referred to as partial ice) immediately before freezing.
2. Description of the Related Art
A water-circulating type ice maker is an ice-maker for producing pure ice where the impurities are eliminated at the surface of the ice-making plate by repeating the process of flowing ice-making water on the surface of the ice-making plate during the freezing cycle. Various kinds of ice makers having different types of ice-making plates such as a vertical flat-plate type, a vertical pipe type, and a spray type are known. When freezing ice-making water, however, all of these water-circulating type ice makers have a common problem: cotton-like or a slushy ice is generated before freezing begins because the freezing temperature of the ice-making water becomes 0.degree. Celsius or less.
The type of ice generated depends upon the flow rate, plate surface temperature, etc. Among the above-mentioned examples, in the case of the spray type ice maker, which has the highest flow rate and a low plate surface temperature, cotton-like ice develops momentarily in the water-gathering pan or the water tank into which unfrozen water falls. Further, in the case of the vertical flat-plate type ice maker which has the lowest flow rate and a high plate surface temperature, slushy ice may develop partially on the ice-making plate to disturb the circulation of the water.
Various methods are conventionally known to prevent the generation of the partial ice, such as slushy or cotton-like ice. For example, Japanese Patent Application Laid-open No. 58-15706 discloses a technique for preventing generation of partial ice by predicting the generation of partial ice when the ice-making water reaches a predetermined temperature and temporarily stopping the supply of ice-making water from the ice-making water tank to the ice-making plate so as to either supercool the ice-making plate or form an initial ice film, and then resuming the operation of the ice-making water circulating pump.
Furthermore, Japanese Patent Application Laid-open No. 6-21753 discloses a technique for preventing generation of partial ice (cotton-like ice) by predicting the generation of partial ice when the ice-making water reaches a predetermined temperature and temporarily closing a solenoid valve or the expansion valve in the refrigerating circuit of the ice maker so as to rapidly lower the temperature of the ice-making plate to form the ice core necessary for ordinary ice growth on the ice-making plate.
Although both of the above-mentioned methods can prevent the generation of partial ice, they have the following problems.
For example, in the case of the former method of stopping the ice-making water circulating pump, since the refrigerating load becomes essentially zero with a long stop time, the liquid refrigerant returns from the evaporator to the compressor without evaporating and there is a risk of liquid compression in the compressor. However, the length of the stop time required to generate liquid compression in the compressor varies depending upon the temperature of the ice-making water at the time of the stoppage, the size of the refrigerating capacity with respect to the freezing load, etc.
When the former technique is applied in an ice maker having a harvesting water tank, the cooled and circulated ice-making water returns to the ice-making water tank during the stoppage of the ice-making water circulating pump without making ice so that the amount of water in the ice-making water tank increases. Then, since any cooled ice-making overflow water flows down into the harvesting water tank, energy is wasted. Furthermore, since the temperature of the harvesting water in the harvesting water tank is lowered due to the overflow water, the harvesting time is prolonged, and the problem of decreased ice-making capacity occurs.
In the case of the latter technique of opening the solenoid valve or the expansion valve, since the ice-making water circulating pump is not temporarily stopped, the above-mentioned problem of the former technique can be solved. However, since the refrigerant is not supplied from the compressor to the evaporator and the refrigerant left in the evaporator is sucked by the compressor, a long stop time involves the risk of stopping the operation of the ice maker due to the low pressure of the refrigerating circuit so that the low pressure switch functions to stop the compressor. The length of stopping time required to cause the operation of the ice maker to halt varies depending upon the temperature of the ice-making water at the time of the stoppage, the size of the refrigerating capacity with respect to the freezing load, etc.
The mechanism of partial ice generation has not been completely clarified. However, according to the knowledge and experience of the inventors, it is understood that when the refrigerating capacity is smaller than the refrigerating capacity necessary to cool and freeze the ice-making water, that is, the refrigerating load for making ice when the temperature is slightly above the temperature where the ice-making water starts freezing, partial ice is generated. On the other hand, when the refrigerating capacity is sufficiently large with respect to the refrigerating load for making ice, partial ice is not generated.
The size relationship between the refrigerating capacity of the ice maker and the refrigerating load for making ice varies depending upon the number of times the freezing cycle has been repeated since starting operation of the ice maker. For example, when operation of the ice maker is resumed after a long stoppage, the temperature of the components comprising the ice maker are high in the initial freezing cycle. Further, the refrigerating load for making ice can be roughly divided into the refrigerating heat quantity needed for cooling the ice-making water and the refrigerating heat quantity needed for cooling elements excluding the ice-making water, such as the components comprising the ice-making water circulation path. Therefore, in the initial freezing cycle, the refrigerating heat quantity needed for cooling the elements excluding the ice-making water is large. Accordingly, the refrigerating capacity is small with respect to the refrigerating load for making ice and it can be said that partial ice can be generated easily.
The relationship between the refrigerating capacity of the ice maker and the refrigerating load for making ice also varies depending upon the surrounding air temperature. For example, the higher the surrounding air temperature, the smaller the refrigerating capacity of the ice maker becomes. Accordingly, the refrigerating capacity easily becomes small with respect to the refrigerating load for making ice, and partial ice can be generated easily.
The relationship between the refrigerating capacity of the ice maker and the refrigerating load for making ice also varies depending upon the frequency level of the power source of the ice maker. For example, when an ice maker compatible with both a 60 Hz power source and a 50 Hz power source is used with the 50 Hz power source, the refrigerating capacity of the ice maker becomes smaller than when it is used with the 60 Hz power source. Therefore, the refrigerating capacity easily becomes small with respect to the refrigerating load for making ice, and thus the partial ice can be generated easily.
Furthermore, as mentioned above, generation of partial ice depends upon the size of the refrigerating load for making ice with respect to the refrigerating capacity of the ice maker. This relationship is particularly important when the ice-making water changes state to ice.
More specifically, since 80 kcal of latent heat is necessary per 1 kg of water for a state change of the ice-making water to ice, a maximum refrigerating capacity is required at the time of this state change. The state change is denoted as the maximum load point. If the refrigerating capacity for cooling the ice-making water is insufficient at the maximum load point, the cooling rate of the ice-making water becomes slow so that the time to pass through the maximum load point increases, and the generation of partial ice becomes likely.
According to the above-mentioned partial ice generation mechanism, partial ice is not always generated when ice-making water reaches a certain temperature, but is generated when the refrigerating capacity is small with respect to the freezing load when the ice-making water reaches the certain temperature.
However, conventional techniques always dealt with measures for preventing the generation of partial ice only when the ice-making water reaches a predetermined set temperature. Therefore, the conventional techniques facilitated the above-mentioned accompanying problems when the counter measures for partial ice generation were not necessary.
As mentioned above, in general, both the refrigerating capacity of the ice maker and the refrigerating load for making ice vary according to the operating conditions. Furthermore, the refrigerating capacity of the ice maker can be theoretically calculated from the cooling heat source such as the external air, and from the temperature of the ice-making water. On the other hand, the refrigerating load for making ice includes the refrigerating heat quantity for freezing the ice-making water and the refrigerating heat quantity for cooling the elements excluding the ice-making water. Among the heat quantities, the former can be calculated theoretically. However, it is difficult to calculate the latter theoretically, and it can only be estimated based on the value obtained from experience or experiment. Thus, it may drastically vary according to the operating conditions. Therefore, it is considered difficult to predict the refrigerating load for making ice, and to determine whether or not the refrigerating capacity of the ice maker is insufficient with respect to the predicted refrigerating load.